MOSFET or DMOSFET transistors are often used as switches, for example, to connect a load to a power supply. It is often required that the switch be placed in series with the positive terminal of the supply, and that the FET switch be "N" polarity. ("N" channel MOSFETs or DMOSFETs will be henceforth referred to simply as NFETs.)
When an NFET is used as above, the Drain is connected to the supply and the Source is connected to the load. To open the switch, the Gate is placed at or below ground potential. To close the switch, the Gate must be driven positive relative to the Source. As the NFET starts to conduct, the Source becomes positive and approaches the potential of the Drain terminal.
The Gate to Source voltage needed to place the NFET in an acceptably low resistance on state is higher than the Drain to Source voltage that results from being in that on state. The Gate terminal must therefore be driven higher than the Drain, or positive supply. A second supply must therefore be provided that is at higher potential than the primary supply.
The second supply is often generated by means of a charge pump. It is often desirable that the charge pump use capacitors that are internal to an integrated circuit. Due to the limited size of internal capacitors, this results in a supply of limited capability.